| 35mm film, showing sprocket holes|
inherited from its origins as movie film
image by Voxphoto (Image rights)
The science of film
The chemical elements bromine, chlorine and iodine are collectively known as the halogens, and all react with silver to form silver halides, which are light-sensitive compounds. Of these, silver bromide (AgBr) is especially light-sensitive, and this is what makes film photography possible. Fine silver halide grains (of controlled size) may be suspended in liquefied gelatin, and coated onto a transparent backing. This is conventionally termed an emulsion—although since it not a mixture of two liquids, this is a slight misnomer.
When light strikes one of these grains, a few bromine atoms are kicked off the surface. Gelatin has the property of immobilizing these atoms—potentially for decades—and the specks of pure metallic silver remaining on the film grains form a latent image.
When the exposed film is bathed in a suitable reducing agent, the metallic silver specks strongly catalyze the reaction converting the silver halide grains into metallic silver. By timing this chemical development correctly, unexposed areas of the film remain pale silver halide; while exposed areas develop as black, due to the fine soot-like metallic silver particles embedded in the gelatin.
Finally, another chemical bath is used which dissolves and removes the unexposed silver halides, which are still light-sensitive. This fixes the image.  Because exposure to light results in a dark area on the film, and vice versa, the resulting image is a negative. This image may be projected onto a similar emulsion coated onto paper. This again reverses the image tones, yielding a normal positive print.
It is also possible to use reversal processing on a latent film image. In this case, it is the silver of the developed negative image that is chemically dissolved. The silver halide grains remaining behind are flashed with light or chemically activated, then given a second development. This creates a dark silver image in the unexposed areas of the scene, and thus it is a positive rather than a negative.
The description above gives the essential principles behind traditional black & white films. An unmodified silver-halide emulsion has its greatest sensitivity to blue light, and little sensitivity to red. But an emulsion can include sensitizing dyes that expand the spectral sensitivity, either just into the green spectrum (for an orthochromatic or "ortho" film) or throughout the visible spectrum (for a panchromatic or "pan" film).
These various color-sensitizing agents make it possible to create color film, by coating multiple layers onto the same support: A topmost silver halide layer can be sensitized only to blue light; below that a yellow filter layer is coated. This blocks all blue light from reaching the subsequent layers. The next silver halide layer is an ortho emulsion, but now only responding to green light. Beneath this is coated a red filter; and at the bottom a panchromatic emulsion, which now only responds to red light.
While these are essentially three black & white emulsions, they have divided the color spectrum into three, and thus can represent the color of a scene in a way that matches the eye's color vision. The image may developed into a color negative, or using reversal processing into a positive color transparency—a color slide.
Current color films incorporate color couplers—compounds that react chemically with the developed areas of the image to yield color dyes. Different types of couplers are used to create dye colors appropriate to each layer of film. After these are generated, the original silver image is chemically dissolved, leaving only the color-dye image behind.
While simplifying processing, these dyes deviate considerably from the ideal in their spectral absorption. Much film manufacturer R&D has gone into managing this issue; and in practice, multiple coating layers are often used. Furthermore, film may be given anti-reflection and anti-curl layers, plus a protective surface covering. The result is that a modern film may be coated with 17 different layers. Kodak's current coating line in Rochester, New York has the capability to coat film with up to 20 layers if necessary.
The larger each individual silver halide crystal is, the greater its cross-sectional area. At any given illumination level, this larger area will intercept more photons, and so render that grain developable into a black speck in the image. Thus, coarser-grained films tend to be higher in film speed. Whether the resulting "grittiness" in the final image becomes objectionable depends on the negative and print dimensions, as well as the aesthetic intent of the photograph.
Manufacturers carefully control the size distribution of silver halide crystals in their emulsions. Newer emulsions use grains where the crystallization has been controlled even more, e.g. to create flattened "tabular" grains with increased cross-sectional area, but which allow more adjacent grains to overlap. This reduces the perceived graininess of the image. These engineered grain shapes have become ubiquitous in modern color emulsions, and common in black & white ones. But some B&W users still prefer the older formulations, which may offer more contrasty "snap," despite their greater graininess. These older emulsions are sometimes termed "cubical" grain films, although in reality their grain shapes are irregular.
In March 1884, George Eastman patented a process for coating a gelatine emulsion onto opaque paper. After development, the processing lab would separate the emulsion layer and transfer it to a transparent support for printing. This required the treatment of the gelatine with chrome alum to make it insoluble in water; it was applied to the paper over a thin base layer of untreated gelatine. Eastman was so certain of the promise of the idea that, based on his 1884 patents, he renamed his Eastman Dry Plate Co. to become the Eastman Dry Plate & Film Co. Soon after, in 1889, Eastman offered his first transparent, celluloid-base film, superseding the earlier "stripping" film. This simpler concept became the basis of all subsequent film photography.
Coating emulsions onto a flexible support was an idea that had occurred to many people. For example, in the same month as Eastman’s 1884 US patent, the British Journal of Photography reports both a British patent, and a public presentation at the London and Provincial Photographic Society of a similar system. This patent was claimed by Joseph Sachs, in association with Fickeissen and Becker of Germany. In this system, the paper base sheet was to be made transparent by coating it with copal varnish (varnish based on tree resin, in a solvent), the surface then smoothed with fine powdered pumice, and, as in the Eastman process, a base layer of gelatine or isinglass applied to allow the gelatine emulsion to adhere. In this process, it was not proposed that the emulsion be removed from the paper base. The editors clearly believe that flexible supports will become common: 'it does not take much acumen to prophesy that for landscape work, one department of gelatino-bromide that will ere long become developed in an extensive manner is that of the employment of paper as a support for the sensitive layer'. They are rather grudging as to the originality of the basic idea, stating ‘we have no doubt that varnished paper has, for the purposes of negative photography, been tried by many’; this was natural at a time when many photographers prepared their own materials, coating and developing their own plates, and even mixing their own emulsions.
Eastman’s real innovation was in extending photography to people who were not able to do this kind of preparation and development, or interested in doing so.
Besides his technical innovation, Eastman was a shrewd businessman with an eye for promotion. In 1888, he loaded 100 exposures of his paper backed roll film into an easy-to-use box camera and gave it the invented and memorable name Kodak. He promoted it with the famous slogan "you press the button, we do the rest." While it would have been impractical to have 100 glass plates shipped to Rochester, New York and back for developing, lightweight roll film made this a viable business model. Although the $25 original price, adjusted for inflation, would equal over USD $600 today, the Kodak was an immense hit, and photography was on its way to becoming a daily part of life worldwide.
- Traditional, cubical grained silver-halide black and white films (Examples: Ilford FP4, Kodak Tri-X)
- Engineered-grain silver-halide black and white films (Examples: Kodak TMax 400, Ilford Delta 100)
- Color negative films (print film) using development process C-41 (Examples: Fujifilm Superia 400, Kodak Ektar)
- Chromogenic black & white films using development process C-41 (Examples: Kodak BW400CN, Ilford XP2 Super)
- Color reversal film (slide film) using development process E-6 (Fujichrome Astia 100F, Kodak Elite Chrome 200)
- Kodachrome reversal film (slide film) using development process K-14 (now discontinued)
Instant-print media such as those from Fujifilm and formerly, Polaroid, are also typically known as "film" packs—although in these cases, the light-sensitive medium is simply one layer in a sandwich of components.
Historically there have been a plethora of film formats and image sizes used in photography. At the small end, there is the film used in the Minox camera which is only 9.2mm wide. On the larger side, view cameras may use sheet film, commonly up to 8×10″ but sometimes up to 20×24″. In the early 20th century, a variety of paper-backed roll film sizes were in use, up to nearly 4 inches in width. Over the years, various easy-loading film formats have been pitched at the snapshooter market, including 126 and 110 cartridges, Disc and APS film.
But two film formats have endured the longest: Perforated 35mm film, originally a movie stock, then later used for still cameras with Kodak's 135 cassette format; and 120, a paper-backed roll film introduced for Kodak's Brownie No. 2 of 1901 (although the designation "120" was not applied until later).
For 120 film, Linhof first introduced 6x7 (56x76mm) backs for their Technika cameras around 1956-1957. The format was announced as the "ideal format" for it's ability to fit standard paper sizes. Before this came the Omega 120 camera in 1954, using the 2 1/4 x 2 3/4 format. Later cameras using the format were the Pentax 6x7 (shown as a prototype at the 1965 Photokina), Mamiya RB67 and Linhof 220.
| 120 film edge markings|
image by Voxphoto (Image rights)
| 35mm edge codes (frame numbering in the 50's from 100-foot bulk roll)|
image by Voxphoto (Image rights)
Film manufacturers typically expose the edges of the film, outside the image area, with a variety of codes and numbers. At a minimum, these will identify the film brand and give a sequence of numbers useful in referencing particular frames on a roll. Many 35mm films will also include DX bar coding, giving the film type and frame numbering in machine-readable form.
In the case of 120 and 220 film, the exact number of frames per roll depends on the format of the camera; hence the index numbering may not correspond exactly with individual frames. Likewise, with 35mm film sold in bulk 100-foot rolls the frame number sequence is arbitrary, rather than starting at zero and counting up to 24 or 36.
Older film may include the label "safety film," which indicates a cellulose acetate base, as distinct from the earlier and highly-flammable nitrate-base film.
When a photographer discovers a film unexpectedly blank after developing, the presence of normal edge codes indicates the failure was with the camera. In contrast, film that is clear even where edge codes should appear has been processed incorrectly—put through fixer before the developer, for example.
Edge codes have an iconic status in announcing "this is film," which may lead a photographer to include them as an integral part of the images they present. Simulated edge markings are frequently seen today as an effect added to digital images—often getting the details completely wrong (all pictures having the same frame number, appearing on the wrong edge, etc.)
- ↑ One chemical compound used for this used to be called sodium hyposulfate, and thus hypo entered the lexicon of photography. Today it is properly called sodium thiosulfate; and ammonium thiosulfate is also used.
- ↑ Rather than using a red filter layer, the bottom emulsion may employ a dye that only sensitizes it to red, not green light.
- ↑ A notable exception to this was Kodak's Kodachrome film; its development added the color dyes to each layer as a separate chemical step during processing—one reason why processing of this film ended in 2010.
- ↑ A traditional black & white film lacks any of these dye couplers. If put through color development processing, it will come out completely clear.
- ↑ Robert L Shanebrook, Making Kodak Film: The Illustrated Story of State-of-the-Art Photographic Film Manufacturing (Rochester NY: Robert Shanebrook Photography, 2010).
- ↑ Eastman's 1884 patent, also available as a PDF, at Google Scholar
- ↑ Kodak history 1878-1929 at Kodak corporate history site
- ↑ In 1913, a court found that Eastman Kodak had infringed on an earlier celluloid-film patent by Reverend Hannibal Goodwin; but it was not commercialized until after his death.
- ↑ British Journal of Photography Vol. 31, No. 1247, pp 193 (editorial: Flexible negatives) and 204 (report of patent: Manufacture of pliable plates and surfaces as a substitutes for glass for photographic purposes, &c.). Whole volume (all of BJP for 1884) available in various formats including PDF at the Internet Archive.
- Part two of History of Photography and the Camera: Improving Films and Prints, on inventors.about.com
- How Much Longer Can Photographic Film Hold on? from Boston.com; essay on future of film.
- Un Mondo d'Argento - Breve storia dei supporti per la fotografia in Italia 1839-1939 (by Donato Consonni - in Italian)